Several things come to mind.
Lighting - clams do well with bright light, but if the light is different, like going from natural sunlight, where the clam was bred and raised, to LED's, it can take some time for the clam to get accustomed to the new light.
Nutrients - In addition to proper light, the large population of zooxanthellae algae in the clams mantle need to eat N pollution.
Positioning - This relates to water flow and substrate. Some clams like to be in the sand, others prefer a depression in a rock while others prefer to connect their foot to a rock buried in up to an inch of sand. Moderate water flow is good but clams can handle a wide range of flow.
Supplemental feeding - Clams actually love to eat phytoplankton. I have used clams to clear a tank of a bloom of green water / single celled algae. I buy Phyto Paste to feed my clams.
We used to hear the experts talk about how a skimmer is not needed as much where clams are present. I take that to mean that over-skimming is probably not so good if a healthy well fed clam is wanted. Back in the day when Tridacna clams were not so expensive, I kept some very nice large ones in skimmerless tanks.
I believe there was a thread about this not so long ago... yeah here it is:
Aloha,
Mark